But not on AMD machines from 2012 and before. You can buy a high-end motherboard (KGPE-D16) that can run libreboot and 2 16core Opteron 62xx cpu's with 192GB ram. You don't have to go the old and relatively slow thinkpad route to achieve freedom.
I’m interested in something like this (though maybe not as beefy) for self-hosting.
Are there any companies that sell the gear you describe, or guides/wikis on getting set up?
If you are on a tight budget however i recommend buying the motherboard on aliexpress (for about $200) and if you are not comfortable in flashing libreboot yourself you can ask a company that delivers bios chips for this motherboard to flash a custom bios and supply the libreboot rom binary to them. You can easily swap the bios chips yourself. The libreboot and coreboot websites have lists of hardware that are compatible (ram/cpu).
Also if you are interested in newer liberated hardware, look into the Talos II. They provide a proper workstation that comes with only free software. It will be more difficult to setup since it has a different cpu architecture, but it is definitely the way forward.